What to do if my father passed away and his live-in girlfriend at the time got his Will probated?

You have not provided enough information to give you a good answer to your question.  If your father's will named his girlfriend as the "executor" of his estate, then she absoultely would have the right to probate his will.  And if he named her as the "beneficiary" (the person who should receive his assets), then he was free to do so as long as she did not "unduly" (unfairly) influence him, threaten him, or defraud him.